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redhatcveRedhat.comRH:CVE-2021-4155
HistoryMay 07, 2022 - 2:17 p.m.

CVE-2021-4155

2022-05-0714:17:19
redhat.com
access.redhat.com
103

5.5 Medium

CVSS3

Attack Vector

LOCAL

Attack Complexity

LOW

Privileges Required

LOW

User Interaction

NONE

Scope

UNCHANGED

Confidentiality Impact

HIGH

Integrity Impact

NONE

Availability Impact

NONE

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N

0.0004 Low

EPSS

Percentile

5.1%

A data leak flaw was found in the way XFS_IOC_ALLOCSP IOCTL in the XFS filesystem allowed for size increase of files with unaligned size. A local attacker could use this flaw to leak data on the XFS filesystem otherwise not accessible to them.

Mitigation

This issue can be mitigated by ensuring xfs_alloc_file_space is not called with "0" as an argument.

This can be done with a SystemTap script (which resets "0" with XFS_BMAPI_PREALLOC), below are the steps:

  1. Save the following script in a 'CVE-2021-4155.stp' file

-– On Red Hat Enterprise Linux 6 —
probe module("xfs").function("xfs_alloc_file_space") {
if ($alloc_type == 0)
$alloc_type = 0x40; # XFS_BMAPI_PREALLOC
}
-– On Red Hat Enterprise Linux 6 —

-– On Red Hat Enterprise Linux 7 onwards —
probe module("xfs").function("xfs_alloc_file_space") {
if ($alloc_type == 0)
$alloc_type = 0x8; # XFS_BMAPI_PREALLOC
}
-– On Red Hat Enterprise Linux 7 onwards —

  1. Install systemtap package and its dependencies

    yum install -y systemtap systemtap-runtime

    yum install -y kernel-devel kernel-debuginfo

  2. Build the mitigation kernel module as root.

    stap -r uname -r -m cve_2021_4155.ko -g CVE-2021-4155.stp -p4

  3. Load the mitigation module as root

    staprun -L cve_2021_4155.ko

What is SystemTap and how to use it?
<https://access.redhat.com/solutions/5441&gt;

5.5 Medium

CVSS3

Attack Vector

LOCAL

Attack Complexity

LOW

Privileges Required

LOW

User Interaction

NONE

Scope

UNCHANGED

Confidentiality Impact

HIGH

Integrity Impact

NONE

Availability Impact

NONE

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N

0.0004 Low

EPSS

Percentile

5.1%